Polarimetric radar (radar which has the ability to transmit and receive in more than one sense of polarization) has uses in weather surveillance and in air traffic control applications. It may also have applications in other areas such as ballistic missile defense. The calibration of polarimetric radars is based, at least in part, on the electromagnetic reflection characteristics of planar and spherical targets to incident circular polarization. A reflective planar target returns the opposite sense of both linear and circular polarization, and a spherical target returns the opposite hand of circular polarization relative to the incident polarization. If a radar system transmits a particular hand of circular polarization, such as right-hand circular polarization (RHCP), raindrops, which are generally spherical, will reflect left-hand circular polarization (LHCP) signals. Assuming that the same antenna is used for radar reception of the returns as for transmission (monostatic radar), or at least assuming that the receive antenna has RHCP characteristics (in a bistatic radar context), the LHCP return signal will tend to be rejected by the receive antenna. Reflection of circular polarization signals, such as RHCP signals, from nonspherical targets are more complex, and do not necessarily simply reverse the polarization, but instead tend to return noncircular elliptical polarization. Thus, an aircraft target will generate reflections in response to incident RHCP which include elliptical RHP together with mutually orthogonal linear polarizations. A RHCP reflected energy receiving antenna will not reject these reflected signals. It should be noted that mutually orthogonal linear polarizations of electromagnetic energy are often referred to as vertical (V) and horizontal (H) regardless of the actual orientation of the electric field.
In the context of weather radar, it is possible to estimate the shape of precipitation by alternately transmitting two mutually orthogonal electromagnetic signals. The return signals from hailstones, which tend to be round, differ from those of raindrops, which tend to be flattened, and these differences can be used to distinguish between hailstones and raindrops.
Thus, there are important uses for radar systems which can transmit selected circular or linear polarizations and selectively respond to particular return signal polarizations. More specifically, weather surveillance and air traffic control radar systems require various forms of polarization diversity, including (a) transmission and reception of circular polarization, (b) transmission of circular polarization and reception of orthogonal linear polarizations and (c) transmission of ±45° slant polarization and simultaneous reception of orthogonal linear polarizations. Array antennas capable of transmitting diverse polarizations are known. In such array antennas, each antenna element includes a pair of crossed linear radiating/receiving elements such as crossed or mutually orthogonal dipoles. Those skilled in the art view such crossed radiating/receiving elements as being a single elemental antenna of the array. The individual crossed radiating-receiving elements are referred to herein as “radiators” regardless of whether they are operated in a radiating or receiving mode, or both.
Unfortunately, the imperfections of antennas and real systems tend to work against the use of polarimetric radar. It is difficult, if not impossible, to make an antenna which transduces only a particular polarization to the exclusion of other polarizations, and this difficulty is compounded by the high power which a transmit antenna must handle. An aspect of this difficulty lies in the precision with which the antenna itself can be fabricated, and another aspect lies in the associated electronics, beamformers, and cables which interconnect elemental antennas of an array antenna, and especially the transmit module which is associated with antenna elements or element subgroups in an active array antenna.
One possible way to adjust the transmitted polarization in the context of an array antenna is to adjust the phase and amplitude of the signal applied to each transmit/receive radiating element of the elemental antenna relative to those of other elemental antennas, so that the polarization of the resulting combined far-field radiation, in a particular direction, meets the desired standard. It is difficult to separate out the far-field contributions of any one elemental antenna, so the correction applied to a given antenna element may be such as to cause that particular antenna element to be far from the desired polarization even though the polarization of the sum radiation is correct. This has the effect of tending to degrade the sum polarization at other aspect angles. Additionally, the correction of phase and amplitude in a phased-array antenna is ordinarily accomplished by digital adjusters, which have fairly coarse adjustment steps. The coarse steps make achieving the desired polarization more difficult than if continuous adjustment were possible. Extremely fine adjustments of amplitude and phase may be possible, but may be unacceptably expensive.
Improved polarimetric radar systems are desired.